A lot of folks have made some exceptional points on this thread. I believe that a singer should be judged not only on their technical abilities (range, power, diction, etc.), but also on their ability to convey the emotions and messages of their music. I'll admit that I appreciate some singers for their technical skills alone (Timo Kotipelto, Fabio Lione, Ralf Scheepers, etc.) and others just for their strong emotional output (Ronnie James Dio, Andy B. Franck, Jon Oliva, etc.). Of course, the singers at the top of my list are those who excel on both fronts (Russel Allen, Daniel Gildenlow, Jorn Lande, etc.).
Another big test for me is whether or not a singer can perform well live. Out of the singers I've mentioned thus far, I've seen all but Fabio live (though I've heard live recordings), and I've been impressed with each one. On the other hand, there are vocalists who I really liked until I heard them live. Some examples are Eric Adams, Andre Matos, and Pasi Rantanen.
I would like to add that I am also a big fan of John West's voice, particularly his work on the first six Artension albums and his solo work on Shrapnel. I would love to see him perform these songs live.
Stay metal. Never rust.
Met-Al